The use of non-modifiable files has increased with the growth of the internet. One standard file format for publishing documents is the Adobe Acrobat file format (generally associated with the extension .PDF). Other file formats include JPEG and bitmap files. A benefit of these files is that one can use these file formats to exchange information on the internet with minimal concern for modification of the documents.
Another file format that is commonly used is the MRC or mixed raster content file. This file format separates an image into three equally sized images and stores them at different resolutions and, in some cases, with different compression rates or technologies. FIG. 2 shows a typical process for generation of MRC files. An image generator 301 outputs a source image 302. Source image 302 has an image resolution of 300 dpi. The source image 302 is separated by a separation process 303 into three separate images 304–306. The three separate images include a black/white (B/W) mask image 304, a foreground image 305, and a background image 306. The three images 304–306 have the same dimensions (x,y) as the source image 302. As shown in FIG. 2, the resolutions of the three images 304–306 are shown as 300 dpi, 300 dpi, and 100 dpi, respectively. The three images may be selectively compressed by compression process 307 to various degrees by different compression technologies.
Despite the benefits of being able to decompose a source image 302 into various other files 304–306, MRC files suffer from a long decomposition process (separation process 303), as is known in the art. The separation process 303 attempts to analyze all components of an image and separate out aspects for inclusion in each of the files 304–306. A need exists to generate MRC files without the long delay.
Also, the composite files 304–306 may be large while having little content. FIG. 3 shows a sample conventional MRC file. The file includes three layers including background layer 401, selector layer or black/white or B/W mask layer 402, and foreground layer 403. For purposes of illustration, the composite image 411 is shown as the resultant image when the three layers (401–403) are combined. As is known in the art, background layer has a color depth per pixel of 24 bits per pixel (24 bpp), selector layer has a color depth per pixel of 1 bpp, and foreground layer has a color depth of 24 bpp.
Each layer comprises one image. Each image has the same dimensions (x,y) (as shown in FIGS. 2 and 3). For example, background layer 401 includes one image with content 404 in the center. Selector layer 402 includes one image with three sets of content 405–407. The content 405 is “Black Text,” which is intended to represent black text. Content 406 is “Color Text A,” which is text that has color A in composite image 411, yet is represented in selector layer 402 as black text. This is because selector layer 402 only has a 1 bpp color depth. Similarly, content 407 is “Color Text B,” which is text that has color B in composite image 411, yet is black text in selector layer 402. Foreground layer 403 includes three sets of color palettes including black 408, color A 409, and color B 410. A concern with the MRC file of FIG. 3 is the significant amount of unused space. The unused space increases the underlying image size of each layer. This unused space increases time needed for encoding, decoding, and composition stages. Also, the combination of unused space with content sometimes results in poor compression of the layers. Accordingly, a need exists to minimize unused space in the composite files 304–306.